The invention relates to a compensating chuck for machine tools which makes it possible to grip a workpiece mounted between centers even on slightly eccentric surfaces without deteriorating the same or exerting undesirable bending moments on the workpiece. This is in particular necessary when the workpiece is mounted between centers, so that these determine its rotation axis during a particularly precise machining step, whilst the workpiece must simultaneously be griped by the collet of a chuck in order to transmit a sizeable moment to it e.g. to perform an eccentric grinding. Because of unavoidable allowances of the surface of the workpiece that is griped by the collet, one must expect that the rotation axis of the colletxe2x80x94simply called chuck axis hereafterxe2x80x94will not coincide exactly with the actual rotation axis of the workpiece, which axis is dictated by the centers.
In order to avoid an uneven pressure of the jaws of the chuck on the surface which they grip, and the undesirable forces which this generates, the radial positions of the jaws must automatically respond to a possible eccentricity of the surface of the workpiece which they clamp. Balancing chucks with so-called compensating jaws are known which are built so that their jaws need not all have the same radial distance from the axis of the jaw because the chuck automatically equalizes the clamping forces exerted on the jaws. This can be achieved e.g. by using for the transmission of the clamping forces a hydraulic system which is geometrically identical for all jaws, but it can also be obtained through the use of a wobble plate which evenly distributes the clamping force over all jaws. However, a mere radial adaptation of the position of the jaws to the eccentricity that possibly exists between the rotation axis and the chuck axis neglects another problem that becomes ever more important because of the presently extremely high requirements with regard to the precision of the machining. This is because it corrects the radial distances between the jaws and the chuck axis, but does not eliminate the fact that due to the eccentricity the jaws do not point precisely towards the chuck axis. In practically all cases the jaws grip a cylindrical surface of the workpiece, and as long as the gripping surfaces of the jaws are plane it suffices to correct the radial positions of the jaws for satisfactorily clamping the workpiece even when the axis of its cylindrical surface does not coincide with the chuck axis. This is because the plane gripping surfaces of the jaws rest tangentially on the cylindrical surface of the workpiece even if due to the eccentricity the contact lines between the jaws and the cylindrical surface are not evenly distributed over 360xc2x0, when seen in a transversal cut.
However, in order to avoid excessive local pressures, and hence a possible damage to the cylindrical surface of the workpiece, it becomes ever more usual when a high precision is required to shape the gripping surfaces of the jaws as concave, cylindrical surfaces which are complementary to the convex surface of the workpiece and have practically the same radius. The aim of such a shape is to avoid an excessive local pressure of the jaws on small portions of the workpiece.
This, however, leads to the following problem. If due to an existing eccentricity the central plane of a jaw does not contain the chuck axis but passes outside the same, then the concave gripping surface of the jaw does not fit smugly on the corresponding convex cylindrical surface of the workpiece to be machined; when the chuck is being tightened, a lateral edge of the jaw that is parallel to the chuck axis hits the cylinder first. This happens because due to the eccentricity (when seen in a transversal cut normal to the chuck axis) a moon-shaped gap which ends in a sharp angle exists between the gripping surface and the cylinder. This produces an asymmetrical, locally very high pressure on the workpiece, at the place where the acute angle presses on it; it results in bending the workpiece between the centers in a way that is noticeable when machining with very high precision. Further, this can generate undesirably high local pressures and produce markings on the surface of the workpiece.
One way to avoid such a tilted sit of the jaws on the workpiece and also the bending and possible deterioration of the same which this entails consists in using so-called rocking jaws. These are additional jaws fitted onto the main jaws of the chuck and able to pivot with respect to the latter around a tilt axis parallel to the chuck axis. This can be achieved e.g. by mounting each additional jaw on a pin parallel to the chuck axis so that the additional jaw can tilt around the pin. Rocking jaws are mainly used when the jaw pressure exerted on the workpiece must be distributed as evenly as possible across the periphery of the workpiece, particularly when it is thin-walled. In this case one preferentially uses rocking jaws which extend tangentially fairly far away from their pins in both directions and which have a surface looking towards the workpiece that is shaped so that it rests on the same only near its extremities, that is at two places which lie as far away from the pin as possible in two opposite directions. Thereby the pressure exerted on the workpiece is better distributed over its periphery, e.g. when the chuck has three jaws the pressure is distributed over six places instead of only three when using rigid jaws. The tilting automatically distributes the pressure of each main jaw automatically and evenly over both support surfaces of the rocking jaw mounted on it.
As a side effect, rocking jaws also correct a possibly inadequate orientation of the main jaws towards the chuck axis, because as each rocking jaw comes to rest on the workpiece it orients itself according to the surface of the workpiece, which avoids a skew seat of the jaws on the workpiece. However, this entails several drawbacks if the chuck is intended for an extremely precise machining. More and more often such chucks are hermetically sealed and filled with oil so that the journals of its mutually movable parts are enclosed in an oil bath which protects them against dust, grinding residues and the like. This reduces their wear to a nearly unmeasurable minimum. But the rocking jaws and the journals of its rocking pins necessarily lie outside the oil enclosure of the chuck. Therefore, the unprotected journals of the rocking pins which are not immersed in an oil bath are subjected to more intensive dirtying, which generates an increased wear and premature play. In addition, the rocking jaws and their journals are additional, movable pieces inserted between the main jaws and the workpiece, which is detrimental to the precision of the chuck. Further, the comparatively small dimensions of the rocking pins necessitated by the overall space available make it difficult to obtain a minimum of wear. For these and other reasons, rocking jaws are not used in high precision chucks in order to avoid a skew fit of the jaws on the workpiece.
It is an aim of the invention to propose a chuck which avoids a skew fit of the jaws on a eccentrically mounted workpiece, and the elements of which have journals that can be enclosed within the sealed cavity of a chuck, and which allows to use particularly large pivoting surfaces which suffer a minimum of wear. The chuck of the invention further comprises comparatively few parts because its main jaws fulfil two tasks simultaneously, to wit the gripping of the workpiece through a radial movement, and the swivelling required in order to avoid a skew fit.